We have recently identified a patient who had a false positive low dose dexamethasone suppression test (DST) that likely resulted from treatment with a new insulin sensitizing agent, troglitazone (Rezulin). Data from clinical studies suggest that troglitazone induces hepatic CYP3A4, the most prevalent drug metabolizing enzyme in the liver and the enzyme that is also responsible for the metabolism of dexamethasone. We suggest that as troglitazone becomes more frequently used and as its indications for use broaden, false positive DST in patients treated with troglitazone will become common. The proposed investigations will establish whether troglitazone does alter DST via an increase in CYP3A4 activity.